The so-called metal thin film type magnetic recording medium which has a ferromagnetic metal thin film of metal or Co--Cr alloy formed on a substrate by a vacuum thin film forming technique, such as vacuum deposition, sputtering or ion plating, is endowed with many advantages shown below as compared with the so-called coating type magnetic recording medium which uses magnetic powder and binder.
Specifically, the metal thin film type magnetic recording medium enjoys various advantages such as permitting recording of high density owing to excellent coercive force and high packing density of magnetic material, manifesting a highly favorable electromagnetic conversion property, allowing a notable decrease in the thickness of the magnetic layer, and markedly repressing the loss of thickness during the course of reproduction.
The metal thin film type magnetic recording medium of this nature, however, is generally at a disadvantage in readily sustaining corrosion in the magnetic layer (ferromagnetic metal thin film) thereof and is further at a disadvantage in being deficient in running property and durability.
For eliminating these problems, various attempts have been heretofore made to form a protective film on the ferromagnetic metal thin film. The plasma polymerization film constitutes itself an apt example of such protective film. In the formation of this plasma polymerization film, the feed gas, electric power, surface area of the electrode, etc. must be properly controlled for ensuring impartation of necessary quality to the produced film. As the pertinent prior art, a proposal has been made to the effect that the value of W/(F.times.S), wherein F represents the flow rate of the monomer gas as a feed gas (cc/sec), W the applied electric power (watt), and S the surface area of the electrode (cm.sup.2), is controlled in the range of 5-10(JP-A-61-5435).
The inventors of the present application, as a result of their diligent study on the conditions for the formation of a film by plasma polymerization, have discovered that in the range of the conditions for the plasma polymerization according to the proposal mentioned above, the applied voltage is so large relative to the value of (area of electrode x amount of feed monomer gas) as to entail such problems as frequent occurrences of abnormal discharge and degradation of yield of production or render difficult the operation of an actual production line.
The present invention has been made under such circumstances and has an object to provide a plasma polymerization film forming method and a plasma polymerization film forming apparatus which enjoy advantages such as repressing the occurrence of abnormal discharge to the minimum, permitting the plasma polymerization film to be stably formed for a long time, and promoting the improvement in the yield of production, let alone imparting excellent film properties including durability to the produced plasma polymerization film.